536 THE ORIGIN OF VERTEBRATES 
Thelyphonus, operculum, 189, 190, 206, 207 
Theory, gastreea, 444, 461 
Theories of the origin of vertebrates, 9, 411, 433, 457 
Thionin reaction, 181, 189, 213, 330, 336 
Throat, formation of, 179 
Thyestes, 30, 31, 275, 326, 328, 329, 339, 340, 344 
Thymus, 425, 430 
Thyroid gland of Ammoccetes, 61, 127, 192, 194, 196, 429, 459 
5 » evidence of the, 185 
5 » function of, in Ammoceetes, 213 
Tissues, connective, 471, 474, 481 
54 evolution of, 19 
as notochordal, 435 
5 two groups of, 463 
Tongue of Ammoceetes, 246, 303 
Tonsils, 427, 430 
Torpedo, 262, 392, 470 
Trabecule, 121, 182, 188, 145, 277, 295, 377 
Transformation of the Lamprey, 18, 35, 59, 61, 125, 168, 193, 199, 200, 220, 227, 228, 
287, 291, 804, 807, 809, 381, 336, 347, 349, 389, 445 
Tremataspis, 32, 75, 275, 326, 351, 352 
Trilobites, 24, 25, 26, 437 
4 appendages, 351, 487 
5 diagram of section through a trilobite-like animal, 413 
- dominance of, 26 
+ excretory organs, 396 
i eyes, 74, 88 
‘5 glabellum, 339 
5 relations of, 249, 283 
45 respiratory apparatus, 170 
ventral surface, 437 
Tube ‘of central nervous system, 37, 38, 42, 102, 211, 433, 455, 457 
,, from IVth ventricle to surface of brain in depraoceates, 209 
» Fallopian, 431 
», hypophysial, 229, 244, 317, 440 
,» meeting of four tubes in vertebrate, 318, 440 
», notochord originally a, 436, 440 
,, olfactory, of Ammoccetes, 219, 225, 317, 440 
unsegmented, in segmented animal, 439 
Tanicata, 16 
. budding of, 441 
i degeneration, 12, 17, 19, 60 
‘3 endostyle, 198, 212 
5 hypophysis, 425 
is notochord, 438 
3 position of, 494 
Unit, appendage, in non-branchial segments, 185 
» branchial, 161, 165, 168, 185 
Ureters, nerves of, 448 
Uterus of Scorpion group, 189, 202, 208, 204, 205, 214 
» vertebrate, nerves of, 448 
